The invention relates to a method of operating a multi-cylinder internal combustion engine, wherein some of the cylinders are cut off in part-load operation by an interruption of the fuel supply and by keeping selected valves closed.
By a cylinder or some of the cylinders of an engine being shut down, the fuel consumption can be markedly reduced in part-load operation, especially where large-volume engines are concerned. In this case, the fuel supply to the cylinders to be cut off is interrupted, and the inlet valves and outlet valves of the cylinders are shut down and closed until the refiring of the cylinders. DE 33 15 396 A1 discloses a multi-cylinder internal combustion engine, in which a control device closes only the inlet valve of the cut-off cylinder and the outlet valve of this cylinder continues to be open. It is said that, as a result, the internal combustion engine is operated quietly and smoothly under part-load conditions, since no compression takes place of air which has remained in the cut-off cylinder, such compression being considered harmful. It has been shown, however, that the closing of the gas-exchange valves is advantageous during the cylinder cut-off for achieving as high a fuel saving as possible. The cylinders which continue to provide combustion operate with high compression, while the cut-off cylinders are operated with the valves closed.
During the changeover from the fired operating mode of the cylinders to the cut-off operating mode and back again, jerking may occur, which the driver of a motor vehicle driven by the internal combustion engine feels to be disturbing. The fuel-saving potential depends essentially on how frequently the internal combustion engine can be operated in the operating mode with cut-off cylinders. A high changeover frequency requires a very good changeover quality between operating with all the cylinders and the cut-off mode to provide the highest possible driving comfort.
For a cylinder cut-off for the purposes of fuel saving, in order to achieve smoother running of the internal combustion engine, DE 196 06 402 C2 provides a cut-off pattern in which each fired work cycle is followed by two non-firings which are the result of the cut-off of the fuel supply.
DE 199 07 851 A1 provides a braking engagement of power-operated brakes for the purpose of avoiding a changeover pressure. As an additional measure for avoiding the changeover pressure, an adjustment of the ignition angle and a throttle-valve adjustment, which are to be brought about by the engine control, are proposed.
It is the object of the present invention to provide a method for operating a multi-cylinder internal combustion engine, by which, in addition to achieving as high a fuel saving as possible during the cylinder cut-off operation of the engine, a smooth operating behavior of the internal combustion engine during the changeover between a fired and a cut-off cylinder operation is achieved.
In a multi-cylinder internal combustion engine in which, for fuel saving purposes during part-load operation, some of the cylinders (2) can be cut off by an interruption of the fuel supply to the respective cylinders and by the shutdown and closing of their inlet valves (6) and outlet valves (7) until the re-firing of the cylinders (2) when higher operating loads of the internal combustion engine (1) are required, a smooth operating behavior of the internal combustion engine (1) during the changeover between the fired and the cut-off operation of the cylinders (2) is achieved by an additional compensating opening of the outlet valves (7) irrespective of the gas-exchange control times of the valves for fired cylinder operation.
The invention is based, in this context, on the knowledge that, during a cut-off action immediately after the last fired work cycle, increased power losses occur due to compression work. By virtue of a brief opening of the outlet valve of the cut-off cylinder after the last combustion cycle, the compression work of the piston is reduced and the changeover quality during the cut-off of the cylinder is thereby improved. Additional compensating openings of the outlet valve may take place at least during the first non-fired work cycle. Advantageously the duration of the compensating opening is predetermined by a control unit as a function of the operating state of the internal combustion engine and is increasingly shortened in the following work cycles. As a result, on the one hand, the loss fraction due to compression work is rapidly reduced, whilst at the same time the gas-exchange valves can be transferred into the desired closed position for the purpose of fuel saving without any jerk phenomena. The compensating opening of the outlet valve expediently takes place in a time interval which lies within the crank-angle range of the opening control time of the outlet valve during gas exchange in fired operation. The activation of the outlet valve is simplified when the opening time point of the outlet valve during compensating opening of the valve corresponds to the opening time point in fired operation. The duration of the compensating opening is advantageously defined by a predetermination of the closing time point of the outlet valve as a function of the operating state of the internal combustion engine.
During the changeover from cut-off operation into fired operation, the compensating opening according to the invention results in a quiet operating behavior, in that an over-expansion of the gas enclosed in the combustion chamber is avoided before the first fired work cycle. The compensating opening is advantageously provided instead of the gas-exchange timing of the outlet valve during the normal operation of the internal combustion engine, with the result that the outlet interval is briefly displaced. This considerably reduces the pumping loss which arises during the first opening of the outlet valve within the framework of the gas exchange, when the outlet valve opens near the bottom dead center of the piston movement and under-pressure prevails in the combustion chamber of the previously cut-off cylinder. The compensating opening, which represents the first opening of the outlet valve after the cut-off phase of the cylinder, expediently takes place in a the work cycle crank-angle range which is at about midway in the piston stroke. The opening time point of the outlet valve, which lies in the region of bottom dead center during combustion operation, is displaced by virtue of the location of the compensating opening, so that a lower pressure difference between combustion chamber and exhaust passage is obtained during the following activation of the cylinder than during the opening of the outlet valve in normal operation. Particularly advantageously, the compensating opening takes place during the expulsion stroke of the piston, as a result of which, in the event of the late opening of the outlet valve at midway of the piston stroke after the bottom phase, the exhaust gas enclosed in the combustion chamber can be expelled immediately.
By the omission of the transitional phases between the fired operating mode and the cut-off operating mode, the method according to the invention makes it possible in advance to have an accurate knowledge of the power loss of the internal combustion engine after the changeover action. The torques to be delivered by the individual cylinders can thereby accurately be controlled in advance. It is therefore unnecessary to apply transfer functions in the control unit, for example an adjustment of the ignition point during the changeover phase or provide for similar measures. This allows a substantially more frequent utilization of the operation of the internal combustion engine with cut-off cylinders which is beneficial for low consumption.